Will.
In the days spent within this mine cavern, Shinji had, in truth, already come to understand what it truly was.
As he had reasoned before, what Taigen Sessai spoke of as Truth was the very foundation of one's existence. What you are. Why you exist. Why you take this form and no other. That was the threshold of a daiyōkai. The key to Vaporization. The prerequisite for allowing one's power to expand outward and become a "phenomenon."
What Tōtōsai spoke of as Will was more specific. Narrower. Why do you swing your blade? What do you think of when you swing it? Toward what does each strike point?
The former was why you exist. The latter was why you swing. One looked inward. One looked outward. They returned by different roads to the same destination, yet in truth, they were not the same road.
Shinji had yet to walk either path to its end. But the latter, Will, he felt himself closer to. Because in the human world, he had once heard a term. Kengi. Sword Will.
In the chaos of the Warring States, with demons running rampant, the bodies of humans were, by and large, far less resilient than those of yōkai. Reiryoku was not something everyone could wield. The vast majority of samurai faced against yōkai had only one road, death.
But there were an exceedingly rare few exceptions. Beings called kengō, master swordsmen. Even kensei, sword saints. They possessed no miko's reiryoku. No onmyōji's ritual arts. No blessing of divine artifacts. They had only a single blade. A body of flesh and blood. And yet they could slay yōkai. They could even confront high-ranking demons head-on.
The source of this power, in the martial traditions of the warrior houses, was called Kengi, Sword Will. The understanding of the sword. The conviction in the martial path. The recognition of one's own road. All of it condensed into an intangible force, adhered to the blade's edge.
Shinji hypothesized that, in essence, this was a form of faith power. Yet unlike the momentum that required the convergence of thousands upon thousands, that collective faith-force-field which overwhelmed through sheer quantity, Sword Will was the faith of a single person.
Minuscule in scale. Yet pure to the utmost extreme. Pure enough to cut through yōki.
What was fascinating was that this thing resembled the Truth Taigen Sessai spoke of, yet pointed in the completely opposite direction.
A daiyōkai's Truth was the condensation of the impure energies of the world. A yōkai found its own fundamental essence, and allowed its very existence to expand infinitely, becoming a phenomenon that rewrote heaven and earth. Downward. Deeper. Toward the extreme of the impure. Sword Will was the condensation of the pure.
A human found their own conviction, and allowed their will to continuously contract and coalesce, becoming a strength that transcended the physical body. Upward. Purer. Toward the extreme of the pure. One was the road of the yōkai. One was the road of man.
Shinji was a yōkai. By all norms, he should not possess Sword Will. For a body condensed from impure energies to pursue the conviction of the pure, it was like asking fire to learn from water. The directions were misaligned. The essences conflicted.
But Shinji was no ordinary yōkai. He possessed a human soul. Though it was a dead soul. Yet more than that, it was a soul that had crossed over, retaining the complete human cognition and memories of his former life.
If he could, with that human soul as his core, condense a Will that belonged solely to himself, even if only a half-finished product, then that Will could become a bridge.
A bridge connecting the pure and the impure. A bridge connecting the human soul and the demon soul. When that time came, after he completed the Tenth Transformation, stepped into the Eleventh, used the Shikon no Tama to supplement the Heaven Soul, then comprehended the Man Soul, and completed the construction of the Shura Form's Three Souls, it would all become far more natural, far more logical.
And Muramasa, that demon blade which had waited for him so long, could, under the infusion of this Will, finally complete its last deficiency.
Besides, a yōkai grasping Sword Will was not without precedent. According to Tōtōsai, Tōga had certainly possessed it. Otherwise, his fangs could never have been forged into Tessaiga and Tenseiga, two peerless demon blades, one to kill, one to give life.
Even Sesshōmaru, in Shinji's recollection, having failed to obtain Tessaiga, had ultimately relied upon himself, nurturing within his own body a demon blade no lesser than Tessaiga.
Shinji suspected that the essence of that demon blade was also the materialization of a Will. By that reasoning, those daiyōkai who wielded mighty demon blades and artifacts forged from their own beings, they must all, to some degree, have comprehended it. Rare did not mean nonexistent. And existences capable of becoming daiyōkai were, from the start, vanishingly few. This could even further explain why the vast majority of daiyōkai pursued humanoid forms.
But such a thing, sitting in a volcanic cavern, striking iron, could never be comprehended.
His swordsmanship all along, to put it nicely, was practical combat style.
To put it bluntly, it was brute force and mass-driven devastation. Overwhelming power. Overwhelming speed. Overwhelming control.
In the beginning, he had relied purely on the BowArm Transformation. Now, he relied on the extraordinary arm strength, control, and target-locking of the Soul-Chasing BowArm Transformation, and the explosive speed of the Lightning-Sinewed Body. Fast. Precise. Ruthless. With a lock-on hack. Pure numerical domination.
No Will. No conviction. And certainly no so-called "Sword Will."
So he needed to see. To learn. To fight. To cross blades with true swordsmen. To feel, in those who had spent a lifetime polishing a single blade, what exactly their swords carried within them. And from that, to synthesize, to deduce, and find the piece that belonged to him.
With that thought, Shinji sheathed Muramasa at his waist and rose to his feet. It was time to go. Coming and going, there was nothing to hesitate over.
"Looks like you're heading out, brat."
Tōtōsai crouched beside the half-collapsed furnace platform, his bulging eyes fixed on Shinji as he gathered his things.
"Mn."
"And the sword?"
"Taking it with me."
"It's not yet perfect."
"I know. I'll finish it on the road."
The old yōkai sucked at his teeth. His wrinkled face twisted into an expression somewhere between disapproval and resignation. "Fool… hmph. Finally made a start, and now you're abandoning it halfway."
His mouth scolded, but his hands were already hoisting that iron hammer onto his shoulder, walking toward the depths of the cavern. After two steps, he stopped.
"That hammer of mine, bring it back when you're done. Don't lose it. That's a hammer this old man's been swinging for three hundred years."
"Already returned it. It's by the furnace platform."
Tōtōsai glanced back. The iron hammer was indeed resting perfectly fine against the furnace. He grunted and turned away again. A few more steps. He stopped once more.
"Brat."
"Mn?"
"…Watch yourself on the road."
Those words, coming from that irascible old man's mouth, sounded as though they had been forced out one character at a time from between clenched teeth. Shinji looked at him. He didn't say thanks. He only nodded.
He turned to leave, then remembered something. "Tōtōsai."
"What?"
"Do you know of the Demon Slayer Corps?"
A beat of silence fell within the cavern. Tōtōsai's bulging eyes rolled twice, as though deep in thought. "Demon Slayer… what?"
"The Demon Slayer Corps. An organization dedicated to hunting a type of immortal demon that isn't a yōkai. They use a weapon called a 'Nichirin Blade', forged from scarlet iron sand and scarlet ore. The blade absorbs the power of sunlight and can sever a demon's immortality."
Tōtōsai's expression shifted into deep contemplation. It went from blankness to recollection, his wrinkled face twisting this way and that, his bulging eyes rolling up toward the ceiling for a long while. "Immortal demons that aren't yōkai… Nichirin Blades… scarlet ore…"
He muttered for what felt like an eternity. "Ah!" He slapped his thigh. "This old man remembers now! I had a human apprentice once, no, not an apprentice. Just some brat of a human who snuck in trying to steal my techniques. I caught him, but he refused to leave. Later, this old man got fed up with him, taught him a few superficial tricks, and chased him off. That brat, seems like he was forging 'sun blades' for someone. Kept rambling about how the blade had to be infused with sunlight, how the materials had to be ore that'd been sun-baked for centuries. Foolishness. Sunlight isn't yōki, what use is pouring it into iron? But this old man still pointed him in the right direction, told him how to do it better." He grumbled a few more curses, then paused.
"That brat's skill wasn't bad, to be fair. At least he didn't embarrass this old man."
"Where is he?"
"Last contact was… thirty, forty years ago? Seems he said he was in some mountain north of the Kantō. Which mountain exactly, this old man can't recall." Tōtōsai rummaged through his robes and pulled out a piece of scorched black cloth, then fished out a charcoal stick from who-knew-where.
He scrawled a few crooked lines across the cloth. "Roughly in this direction. Take this and go find him. Show him this cloth if you meet him, he'll recognize this old man's handwriting. If that brat's dead… show it to whoever's around him. The people around that brat will definitely recognize this old man."
He handed the cloth to Shinji. On it was drawn an extremely abstract outline of a mountain, with two lines of text beside it, the handwriting so illegible it was barely recognizable. Shinji tucked it into his robes.
"My thanks."
"Spare me that nonsense. Hurry up and get out of my sight." Tōtōsai waved his hand and turned, disappearing into the depths of the cavern. The sound of hammer blows soon echoed from within, he had already begun packing up to move.
Myōga hopped out from a crevice and stood upon a rock at the cavern's entrance. His six spindly legs pressed together, his flat little body facing the direction Shinji had departed. He watched for a long moment. "As expected of the one who brought 'change' to Young Master Inuyasha…" The flea yōkai's voice was incredibly soft, as though speaking to himself. "That explosion of yōki just now—truly—"
Clang.
A small backup hammer flew out from the depths of the cavern and struck Myōga square on the back of the head. His flat little body was sent flying, somersaulting seven or eight times through the air before crashing down ten paces away.
"Cut the chatter and hurry up and pack! And you're always boasting about having lived in the house of Zhuge Wuhou across the sea, how can you be this slow?! Do you want them to find us again?!"
Tōtōsai's roar echoed out from within the cavern. Myōga lay on his back, all six legs kicking at the sky, trembling for a long while before he managed to flip himself over. "Yes, yes… packing right away…"